POTENTIAL WEED HOSTS FOR DIAPORTHE PHASEOLORUM VAR CAULIVORA, CAUSAL AGENT FOR SOYBEAN STEM CANKER

Greenhouse and outdoor studies evaluated weed species common to Louisi ana soybean (Glycine max) fields as potential hosts for Diaporthe phas eolorum var. caulivora. Species tested were: barnyardgrass (Echinochlo a crus-galli), black nightshade (Solanum nigrum), curly dock (Rumex cr ispus), entireleaf morning-glory (Ipomoea hederacea var. integriuscula ), hairy indigo (Indigofera hirsuta), hemp sesbania (Sesbania exaltata ), ivy-leaf morning-glory (Ipomoea hederacea), johnsongrass (Sorghum h alepense), northern joint-vetch (Aeschynomene virginica), pitted morni ng-glory (Ipomoea lacunosa), prickly sida (Sida spinosa), redweed (Mel ochia corchorifolia), sicklepod (Cassia obtusifolia), smallflower morn ing-glory (Jacquemontia tamnifolia), spiny amaranth (Amaranthus spinos us), tall morning-glory (Ipomoea purpurea), and wild poinsettia (Eupho rbia heterophylla). Soybean cultivars susceptible to stem canker were included as controls. Plants were inoculated twice in each experiment using ascospores in water. Host status of each species was confirmed b y recovering the pathogen from weeds using selective medium or by obse rving production of perithecia. All weed species tested except johnson grass, barnyardgrass, and curly dock were hosts for D. p. var. caulivo ra. Lesions 1 to 2 cm long and less than or equal to 0.5 cm wide were common on soybean, hemp sesbania, and hairy indigo. These consistently yielded mycelium of D. p. var. caulivora. All other weed hosts were a symptomatic.